Light & Energy

1. Light & Energy Unit 4 Chapter 11

2. Christiaan Hyugens (1629-1695) • Described light using a wave theory. • Light propagates through space just like ripples on a pond.

3. Sir Isaac Newton (1643-1727) • Described light via the corpuscular theory: • Light is composed of tiny, discreet packets called “corpuscles.” • More influential, so his theory won out.

4. Thomas Young (1773-1829) • Performed the double slit experiment that proved light existed as a wave.

5. Wave-Particle Duality • Albert Einstein proved light acts as a particle (eventually called the photon). • Light was already proven as a wave • Light behaves like a wave & a particle…at the same time. • Depends on how/what you are measuring.

6. Waving Through Space • All waves travel at constant speed through a medium. • Can change speed if the medium changes. • The speed (V) of any wave is the product of its wavelength (λ) and frequency (ν). • V = λν

7. Peak Amplitude Trough Parts of a Wave

8. Wavelength (λ) • Distance between identical sections on a wave • Measured in some unit of length (usually meters)

9. Frequency (ν – or – f) • A measure of how often a wave passes a point. • Waves per second = Hertz (Hz) • Means “per second” (/s)

10. Speed of Light (c) • All light waves travel at the same speed in a vacuum (space) • Speed of Light = c = 2.998x108 m/s • (671 million mph) • Usually rounded to 3.0x108 m/s • Remember, speed of a wave is the product of the wavelength (λ) and frequency (ν) c = λν

11. Inverse Relationship • Wavelength and Frequency are inversely related • When one increases, the other must decrease • Light with long wavelengths has low frequencies • Light with short wavelengths has high frequencies

12. Calculations • Speed of light is constant • Green laser λ = 532 nm • (1nm = 1x10-9 m) • Rearrange: c = λν to getν = c/λ • ν = (2.998x108 m/s) / (532x10-9 m) ν = 5.64x1014Hz • 564,000,000,000,000 of the laser’s light waves pass a point in one second.

13. Light Spectrum

14. Light & Energy • Photons of different wavelengths have different energies • Calculate energy using E = hν • h = Planck’s Constant • 6.626x10-34 J·s • Can also rearrange using c = νλ • E = hν = hc /λ

15. Energies • Green laser = 532nm • Redlaser = 655nm • Use E =hc /λ • Green = (6.626x10-34 J·s)(2.998x108 m/s) (532x10-9 m) = 3.73x10-19 J • Red = (6.626x10-34 J·s)(2.998x108 m/s) (655x10-9 m) = 3.03x10-19 J

16. Light & Energy • Shorter wavelengths have higher energies • Radio waves = no issue • Microwaves = heat up foods & carry TV signals • Infrared = you are doing it now • Visible wavelengths = no danger • UV waves = sun burn & skin cancer • X-rays = too much is bad  cancer • Gamma rays ( ) = very bad  death by incineration & cancer γ Size of λ

17. Line Spectra • When gaseous atoms/molecules are heated, they emit light • Color is specific to each substance – like a fingerprint • Spectral analysis shows discrete lines Hydrogen Line Spectrum

18. Absorption Spectra When gaseous atoms/molecules are cool, they absorb light Colors absorbed are the same as those emitted when hot!

19. Determining Atmospheric Content We can determine the content of gases between us and distant stars! Starlight is absorbed by cool gas.

20. The Balmer Series (1885) • Lines in Hydrogen’s spectrum known for a long time • Johann Balmer, Mathematician, came up with formula to predict wavelengths from hydrogen • No one was sure why it worked

21. Neils Bohr proposed… • Electrons limited to specific orbits around the nucleus • Explains Balmer’s equation • Predicted more lines, too!

22. Light Emission • Electrons become excited (absorb energy) and “jump” to a higher orbital. • Electrons lose energy and fall back down to a lower orbital. • When electrons lose energy, they emit a photon of light

23. Energy Levels • Since different wavelengths are emitted, we can calculate the difference in energy between orbitals • ΔE = hν = hc / λ